Wavelength-division multiplexing (WDM) has been employed in core networks for over a decade. WDM technology enables signals of multiple wavelengths to be concurrently transmitted over a given optical medium. This has been facilitated by the availability of wideband optical amplifiers that can simultaneously amplify many different wavelengths without distortion. The advantages provided by WDM translate into greater fiber utilization, lower capital expenditures associated with fiber deployment, and reduced costs in repeater stations by eliminating the need to terminate each wavelength along the fiber path. To maximize economic utility, the wavelength density that can be multiplexed onto a given fiber has increased in recent years: 80-wavelength systems are now common in the EDFA band, with 50 GHz frequency spacings between channels in many offerings.
Quantum Key Distribution and Networking—Message security is a critical concern in today's communication networks. Such security is usually provided through cryptography, a process in which message data is convolved with a known key to produce an encrypted message. The level of security varies with the algorithm and the key length, but security can always be improved by changing keys more frequently. In fact, the only provably secure encryption is the one-time pad, in which there is one key bit per message bit, and keys are never reused. For any encryption method, the security of the message is based on the privacy of the keys. Even the one-time pad can be broken if the keys are known to an eavesdropper. Thus, secure key distribution is the foundation of any encryption system. The classic method of key distribution is to generate keys at one site, record them on a physical medium, then transfer them via human courier to both ends of an encrypted message link. Quantum Key Distribution (QKD) removes the risks associated with courier distribution, enabling collaborative generation of secure keys at the endpoints where they are needed. Security of the process against eavesdropping is guaranteed by the no-cloning theorem, when operating in the single-quantum regime. Classic QKD algorithms, such as the BB84 protocol (Bennett and Brassard, 1984) are designed for point-to-point operation between two sites connected by a dedicated optical link. For a community of K users interconnected by optical fibers, K*(K−1) fiber pairs would be needed. Our new approaches offer a much more efficient, fiber-lean, solution for full connectivity. They also provide for dynamic sharing of the QKD bandwidth, allowing rapid expansion or contraction of the QKD rate at individual sites on an on-demand basis.
Quantum Entanglement—Quantum entanglement is a phenomenon relating the quantum states of two or more objects even when these objects are spatially separated. This phenomenon manifests itself in correlation between measurable physical properties of the entangled objects. The simplest example is a pair of polarization-entangled photons. A photon can have either vertical or horizontal polarization. For two entangled photons the polarization of each is uncertain. However, when these photons are sent to distant observers Alice and Bob, polarization measurements performed by them are correlated. That is if Alice observes a vertical polarization for her photon, Bob's photon will have a horizontal polarization or vice versa. While Alice's result is random (she does not know a priori whether her photon is horizontally or vertically polarized), polarization measurements performed by Bob always produce a result correlated with that of Alice. If a sequential stream of entangled photons is delivered to Alice and Bob, such correlation allows them to form a truly random sequence of zeros and ones that could serve as a cryptographic key for secured communication (Eckert 1991).
Creation of the entangled photons for telecom applications—The entangled photon pairs may be created by a variety of processes in which a photon from a source laser interacts with a nonlinear medium (which could be a special fiber or a waveguide structure), such as the parametric downconversion (PDC) process. In this PDC process, a primary source photon with a frequency ω0 is annihilated and a pair of entangled photons with frequencies ω1 and ω2 is created. In fact, each of the entangled photons occupies a relatively broad optical spectrum of the width BPDC centered at ω1 and ω2. Conservation of energy requires that the sum of the ω1 and ω2 is equal to ω0). BPDC could be up to few tens of nm wide (20-40 nm). Such a spectral width makes the photons unsuitable for communication through optical fibers due to deleterious effects of chromatic dispersion. Thus, the photons are filtered, which reduces their bandwidth to about BF˜1 nm (or 125 GHz). To preserve the entanglement, the filters' center frequencies ωF1 and ωF2 must add up to ω0. That is, the entangled photons are equally spaced above and below the frequency of the primary source photon. One way to provide the needed filtering is the use of a wavelength demultiplexer (WDM). An entangled pair enters the WDM through a common port, one photon leaves through a port A (centered around ωA) to a fiber leading to Alice, and, in a similar fashion, the second photon is directed to Bob through port B (centered around ωB), where ωA+ωB=ω0. If another nonlinear process is used instead of PDC, the mathematical relation between the primary source frequency and the frequencies of the entangled photons may differ from that specified above, but a known mathematical relation will exist and the present invention can be used to establish QKD connection topology.